We have focused our studies on the metabolic impact of antifolates on leukemic cells and cell cultures, with particular emphasis on cellular mechanisms of resistance to lipid-soluble drugs and methotrexate. The major studies in the coming year will be: 1. Examination of the biologic consequence of abnormal incorporation of deoxyuridine into DNA following antifolate exposure. 2. The interaction of exogenous thymidine and deoxyuridine relative to antifolate action. 3. Cellular toxicity of methotrexate as determined by the interrelationship between efficiency of drug transport for methotrexate and dihydrofolate reductase levels in mutants that overproduce the enzyme. 4. The effect of combined drug protocols with both methotrexate and DDMP on selection of cells resistant to antifolates. 5. Dependence of antifolate toxicity on the degree of cellular commitment of DNA synthesis. 6. Regulation of specific replication of the gene for dihydrofolate reductase. Study of the biochemical mechanisms underlying the cytotoxic response to antifolates and other chemotherapeutic agents continues to be the general emphasis of our laboratory program.